Nematodes are a very large group of invertebrate animals generally referred to as roundworms, threadworms, eelworms, or nemas. Some nematodes are plant parasites and can feed on stems, buds, leaves, and in particular on roots. Cyst nematodes (principally Heterodera and Globodera spp.) are key pests of major crops. Cyst nematodes are known to infect tobacco, cereals, sugar beets, potato, rice, corn, soybeans and many other crops. Heterodera schachtii principally attacks sugar beets, and Heterodera avenae has cereals as hosts. Heterodera zeae feeds on corn, and Globodera rostochiensis and G. pallida feed on potatoes. The soybean cyst nematode (Heterodera glycines) infests every soybean-producing state in the U.S., with total soybean yield loss estimates approaching $1 billion per year.
Plant-parasitic nematodes change shape as they go through their life cycle. In its juvenile form, the animals penetrate plant roots. The number of juveniles entering the plant root soon after plant emergence can have a dramatic effect on plant growth and development. Plant damage occurs from juvenile feeding which removes cell materials and disrupts the vascular tissue by inducing the formation of novel plant cell types that are associated in a unique feeding organ, the syncytium. Due to the sedentary nature of their parasitism, cyst nematodes need to obtain all their nourishment from one location, in fact, through the contact with the initial feeding cell.
Cyst nematodes infect as second-stage juveniles (J2), which initiate the induction/formation of the syncytium. During this phase, J2s begin feeding on the growing syncytium and then develop into third-stage (J3) and fourth-stage juveniles (J4) followed by the adult stage. Syncytium formation encompasses reprogramming of differentiated plant root cells, and these redifferentiations are accompanied and mediated by massive gene expression changes, which have been documented in diverse research approaches using soybean and the soybean cyst nematode Heterodera glycines (Alkharouf et al., 2006; Ithal et al., 2007; Klink et al., 2009) and probably most extensively in Arabidopsis infected by the sugar beet cyst nematode H. schachtii (Szakasits et al., 2009). Regulatory networks governing gene expression patterns in nematode-infected roots and particularly in the developing syncytium are very poorly understood.
Existing methods for treating or preventing nematode disease include the use of chemicals, pesticides, and fumigants. The use of pre-plant soil fumigants is highly effective in controlling cyst nematodes and other plant-parasitic nematodes. However, the majority of the fumigant-type nematicides is no longer available and is also costly and difficult to apply properly under the prevailing conditions.
Crop rotation has also been used to control nematode disease. Rotating non-host plants can be effective in controlling nematode disease. Unfortunately, these non-host crops are often less valuable. Cover crops grown between the main crops is another alternative management strategy. Ryegrain, barley, oats, sudangrass, tall fescue, and annual ryegrass have been shown to be non- or poor hosts for some nematodes. Using cover crops, however, can be costly because the cover crops occupy space that could be used to grow more valuable crops.
Biological control organisms have also been used to try to control nematode disease in crops. Commercially available preparations of biological control organisms are limited in their use to regions that can support the growth of the control organism. Moreover, the outcome of using one organism to control another is unpredictable and subject to a variety of factors such as weather and climate.
As can be seen, a continuing need exists for the development of methods and strategies to control and inhibit plant nematode invasion.
It is an object of the present invention to develop plants, seeds, varieties and lines that have improved tolerance to nematode infection and resultant effects on plants.
It is another object of the invention to provide methods for controlling nematode infection that are environmentally friendly and do not rely on chemicals, biological control organisms, or crop rotation.
It is yet another object of the invention to provide novel plant genetic engineering strategies to ascertain more about the mechanism and plant response to nematode infection, to develop resistant varieties and to modulate expression of key components of regulatory pathways that inhibit nematode infection and its affects in the plant.